Optical fiber has found widespread application as a long haul transmission medium for voice and data transmission. For instance, substantially all of the newly installed capacity in the trunk portion of the public switched telephone network in the U.S. is optical fiber-based.
Although optical fiber at present is not widely used in the feeder and distribution portion of multi-user networks, e.g., the telephone network, extension of the use of optical fibers into this portion of networks is desirable and is expected to occur within the near future, resulting ultimately in all-optical communication systems.
Since typically the equipment and labor costs for connecting a subscriber, or a group of subscribers, to a central office or other switching station is a major portion of the total cost of a communication system, the ability to provide such connection by optical means at relatively low cost is of utmost significance, if an optical distribution network is to become commercially feasible. Such a distribution network would be highly desirable since, inter alia, it would be immune to electromagnetic interference and be relatively secure. Since optical fiber can have very large bandwidth, such a network also could in principle, provide to subscribers very wideband communication channels.
The fiber of choice for long-haul applications is currently silica-based single mode fiber, with the operating wavelength typically being about 1.3 .mu.m. Because silica-based fiber generally has lowest loss at about 1.55 .mu.m, it is expected that future long-haul fiber systems will be operating at that wavelength. Necessary components such as radiation sources (lasers) and detectors for use at 1.3 .mu.m are commercially available but are still relatively expensive, whereas the components for use at 1.55 .mu.m are still under development. On the other hand, components for use at shorter wavelengths, e.g., about 0.85 .mu.m, are readily available and quite inexpensive.
Due to the still high cost of sources and detectors for 1.3 .mu.m radiation, it appears that an optical fiber distribution system that operates at 1.3 .mu.m would at present not be economically viable. On the other hand, it would be highly desirable to use in distribution systems optical fiber that is single mode at 1.3 .mu.m and/or 1.55 .mu.m, since this would permit upgrading, without replacement of the optical fiber, to one or both of these wavelengths at a later time when improvements in component costs make this economically feasible. Such upgrading would be desirable, inter alia, because of the resultant decrease in loss, and because both long-haul and distribution portions of the network would then operate at the same wavelength, resulting in decreased complexity and in economies of scale.
Several authors have considered the implications of the use of optical fiber that is single mode at 1.3 .mu.m and/or 1.55 .mu.m (i.e., that has a cut-off wavelength .lambda..sub.c less than about 1.3 .mu.m) in a communication system that operates at a wavelength .lambda..sub.o less than .lambda..sub.c (e.g., at about 0.85 .mu.m). See, for instance, R. Ries, Electronics Letters, Vol. 23 (2), pp. 71-72 (1987). The general conclusion is that the presence of higher order modes results in a very significant decrease of the attainable bandwidth of such a system (to be referred to herein as a "hybrid" system) due to differential mode delay. M. Stern et al, Abstracts of papers at OFC/IOOC 1987, Reno, Nev., Paper MD 2, teach that the bandwidth of a hybrid system can be increased if a section of a second fiber that has a cut-off wavelength .lambda..sub.c '&lt;.lambda..sub.o is inserted into the transmission path ahead of the radiation detector. Since only the fundamental mode can propagate substantially without loss in the second fiber, the second fiber acts as a mode filter that removes the higher order modes from the transmission path.
Although the insertion of a length of an appropriate second fiber does improve the attainable bandwidth of a hybrid system, the approach does have several shortcomings. For instance, it lacks selectively in that it results in simultaneous removal of all higher order modes. However, for at least some applications it would be desirable to be able to selectively tap any predetermined mode (including the fundamental mode LP.sub.01) from a fiber, without substantially attenuating the other modes. Use of one or more second filters also makes later upgrading to operation at 1.3 .mu.m or 1.55 .mu.m more difficult, since it requires removal of the second fibers and, typically, some reconfiguration of the network. Furthermore, use of second fiber mode filters results in considerable loss of signal power, since not only all the power in the higher order modes is stripped from the fiber, but also a significant amount (typically about 1.5-2 db) of LP.sub.01 power is lost at each transition from the first to the second fiber, due to the unavoidable mode field radius mismatch between the first and second fibers. Furthermore, modal noise generated at splices and connections potentially has a deleterious effect on systems operations, especially if single frequency laser sources are used.
In view of the potential significance of an inexpensive "hybrid" optical fiber communication system that has relatively wide bandwidth and can easily be upgraded to single mode operation at longer wavelengths, a hybrid system that uses a mode stripping technique that is not subject to the above discussed shortcomings of the prior art is of considerable interest. This application discloses such a system.
A known approach to tapping radiation from an optical fiber comprises introducing a spatially periodic "grating" (e.g., a mechanical deformation) into the fiber, with the periodicity (and possibly other parameters of the grating) chosen to result in conversion of guided modes into higher order unguided modes. See, for instance, U.S. Pat. Nos. 3,931,518, and 4,253,727, incorporated herein by reference. Other relevant patents are U.S. Pat Nos. 3,891,302 and 3,982,123.
Although the grating technique has generally been considered to be applicable only to multimode fiber, it has recently been discovered that it can, through judicious choice of parameters, also be made to work efficiently with single mode fiber. See U.S. Pat. No. 4,749,248 incorporated herein by reference. Briefly, it is taught there that in the continuum (as a function of propagation constant .beta.) of radiation modes of an optical fiber there can exist certain relatively narrow ranges of .beta. in which constructive interference between the modes occurs, making possible efficient (resonant) coupling between a given guided mode and a given one of these so-called "tunneling leaky" (TL) modes. For background on the leaky mode description of the radiation modes, see, for instance, A. W. Snyder et al, Optical Waveguide Theory, (1983), especially pp. 487-541.